This is a movie I was prepared to like because Robert Downey is an excellent actor and Zack G has proved to be a weird character who can be very funny in the right film.
But, it just doesn't work very well. This story has been done before, and with much better effect, in Planes, Trains, and Automobiles. The two movies have a very similar story arc, but in the one case you have two very funny men playing actual mature adults who have differing personalities, and in the other you have two men who are immature imbeciles with differing personalities. The former are likable, while the latter are just annoying.
Zack G's character is just too bizarre to be credible, but at least you know where they are going with him. Downey, on the other hand, is an excellent actor badly miscast here. His demeanor is just too "dark" and his personality too brooding and cranky to play the part of a good family man trying at all costs to get home in time for his child's birth. To be honest, his character simply isn't very likable, and is sometimes just downright mean, even spitting in an innocent dog's face. This is in stark contrast to the frustrated everyman played by Steve Martin in P,T,&A - a person every traveler (stuck next to an obnoxious slob) understands implicitly. Another example of such a role being done to perfection is Jack Lemmon in The Out of Towners.
I suppose it didn't help that the theater was packed with very young children who had no business going to this movie. The very crude language, extensive drug imagery, and graphic masturbation scenes, while funny for adults, were grossly inappropriate for 5-12 year olds in the crowd. Some people clearly are not cut out to be parents.
Perhaps the most memorable line in the movie was Downey proclaiming that he had never done drugs in his life. Given Downey's well documented real-life problems, his statement in the movie couldn't help but make the audience compare the character to reality, thus serving to knock down the "4th wall" in the process. No doubt the contrast was created by intent, but I'm not sure it was a great idea.
As Steve Martin, John Candy, and Jack Lemmon proved, superior comedy and deeper laughs come from brilliant acting and comic timing. If you have to resort to endless gutter language, potty humor, and masturbating canines, you have already lost the game before it starts.